The term "lithosphere" refers to the outer layer of the Earth, comprising the crust and upper mantle, and extending to a depth of 50 to 70 kilometers. The traditional view of tectonics (changes in the structure of the Earth's crust) is that the lithosphere consists of a strong brittle layer overlying a weak ductile layer, the system producing two forms of deformation, namely, brittle fracture in the upper layer (accompanied by earthquakes), and aseismic (without earthquakes) ductile flow in the lower layer. The current consensus is that this view is generally correct but imprecise, since the accumulated evidence is now interpreted to indicate that frictional events along fault lines, rather than new fractures, are the causes of earthquakes. The essential idea is that fault lines, which are the interfaces between the crustal plates, build up stresses resulting from the movements of the plates, and at intervals these stresses are suddenly relieved by interface slippages the surface manifest- ations of which are earthquakes. In mechanics, "stick-slip" friction is friction between two surfaces that are alternately at rest and in motion with respect to each other, and in recent years a number of laboratories have conducted model experiments with stick-slip rock systems with the idea of obtaining a fuller understanding of the physics of frictional phenomena occurring at fault lines. C.H. Scholz (Columbia Univ., US), in a review of current ideas concerning earthquake mechanics, points out that at present the most precise and predictive model for earthquake mechanisms is that an earthquake is a frictional rather than a fractional phenomenon, with brittle fracture of the upper litho- sphere layer playing a secondary role in the lengthening of faults and frictional wear. The origin of earthquakes is evid- ently a stick-slip frictional instability, and many of the aspects of earthquake phenomena can apparently be explained by the general laws applying to frictional stability regimes.
QY: Christopher H. Scholz, Columbia Univ., Dept. Earth and Environmental
THE PREDICTION OF EARTHQUAKES
Earthquake prediction, an aspect of geophysics of obvious tremendous social and economic importance, demands from geophysicists more than they are presently able to give. Seismicity patterns, in conjunction with knowledge of where historic earthquakes have occurred, permit reasonable judgments of where future earthquakes are most likely to occur, but at present it is not possible to predict when an earthquake is likely to happen in an endangered area. And of course it is the when that is of great social and economic and even political importance. A recent published exchange of letters among seismologists focuses on the problems of earthquake prediction, the exchange provoked by a previous article which emphasized that such predictions are not possible (R. J. Geller et al, Science 275:1616 1997). Max Wyss (University of Alaska, US) suggests that research in the physics of preparation for catastrophic rupture should not be halted, and that if the lack of funding for earthquake prediction research continues in the US, the important discoveries will be made in Japan, Europe, or China. Richard A. Aceves and Stephen K. Park (University of California Riverside, US) suggest that the review by Geller et al is "an unduly negative view of research in a difficult field." But these authors admit it is time for present earthquake prediction research to be more honestly identified as earthquake monitoring. They suggest, however, that considering the large benefit if and when such research will bear fruit, earthquake prediction research should definitely continue. Robert J. Geller et al (4 authors at 3 installations in JP, US, IT), the authors of the review that provoked the letters, respond that they believe emphasis should be placed on basic research in earthquake science, real-time seismic warning systems, and long-term probabilistic earthquake hazard studies.
QY: R. Aceves
DEFORMATIONS IN THE SAN ANDREAS FAULT LOWER CRUST
A geophysical fault is a break in rock structure that occurs when pressures in the Earth's crust are strong enough to cause fracture and displacement, and earthquakes are common at such break points. Seismic velocity refers to the propagation velocity of a seismic disturbance (e.g., an earthquake), and reflectivity cross-section is a parameter associated with the reflective properties of a propagated seismic wave. The Mohorovicic Discontinuity (called "Moho" and named after Andrija Mohorovicic, who first identified it in 1909) represents the boundary between the crust and mantle, its depth varying from about 5 kilometers to as much as 60 to 80 kilometers. A strike-slip fault is a movement parallel to the fault plane, and the San Andreas fault of California is of this type. Continental drift is the slow movement of the Earth's land masses, a shifting across the underlying molten material, and sea-floor spreading is the process whereby sea floor is continuously created as the crustal plates move apart and continuously destroyed where the plates push against each other. And finally, plate tectonics is the modern theory that unifies many of the features and character- istics of continental drift and sea-floor spreading into a coherent model. Timothy J. Henstock et al (3 authors at 2 installations, US) now report that analysis of a continuous seismic velocity and reflectivity cross-section of the San Andreas fault system in northern California reveals offsets in the lower crust and the Mohorovicic Discontinuity near the San Andreas and Maacama strike-slip faults, and that the northern California continental margin to the eastern edge of the Coastal Ranges is underlain by a high-velocity lowermost crustal layer that may have been emplaced within 2 million years following the removal of the plate slab known as the Gorda plate. The authors suggest that the rapid emplacement and structure within this layer are difficult to reconcile with existing tectonic models.
QY: T. Henstock, Rice Univ., Geol. and Geophys. (713) 527-4880)